A membrane filtration apparatus, such as is exemplified in FIG. 2, has a structure in which a membrane element 1, formed with a large number of filtration channels 12 arranged in a porous portion 11 so as to be parallel to one another, is accommodated in a casing 13, as shown in FIG. 4. The membrane filtration apparatus performs a filtration operation as follows. In a cross flow filtration method, raw water a is introduced into a primary inlet chamber 13a. The raw water a is then passed through the filtration channels 12, during which time it is filtrated by minute filtration pores within the porous portion 11 and the raw water then circulates via a primary outlet chamber 13b and an upper header pipe in the form of returned raw water b. The water that has been filtrated is then taken out from a header tube 14. In a dead end filtration method, the primary side of the apparatus is filled with water, and then, the valve located at the upper portion of the casing is closed. The raw water that has been introduced is processed and the entire amount thereof is obtained as filtrated water via a filtrated water header pipe 14. The water c that has been filtrated is taken out from the secondary chamber 13c located between the side surface of the casing 13 and the side surface of the filtration element 1.
As the filtration operation proceeds, suspended solid (SS) that has been separated accumulates on an inner surface of the filtration channels 12 so as to develop into fouling substances which result in a diminution in filtration efficiency. In such circumstances, the filtration operation is stopped at an appropriate timing and a backwashing operation is performed whereby backwashing water f is supplied from the secondary chamber 13c at a high pressure so as to flow back through the filtration pores of the porous portion 11, and peel off fouling substances that have accumulated on the inner surface of the filtration channels 12. After the backwashing water f has peeled off the fouling substances, it is removed in the form of a backwashing waste water e.
In such a circumstances, the filtrated water c itself is normally utilized as a backwashing water f. However, as the backwashing operation is repeated, the volume of backwashing waste water e increases, whereas the volume of water c filtrated decreases relatively. As a result, a problem arises insofar that the operational efficiency of the filtration apparatus diminishes. In an attempt to solve this problem, a method has been suggested in which, instead of utilizing filtrated water c as a backwashing water f (Patent Exhibits 1 and 2), high-pressurized air is supplied for performing a backwashing operation.
However, in both of these cases, one where the filtrated water is used, and the other where high-pressurized air is used to effect the backwashing operation, a large amount of water that has been lodged in the membrane element 1 is suddenly pushed toward the primary side at one and the same time. In order to enhance the backwashing effect, it becomes necessary to expel the backwashing wastewater from the system quickly. To this end, it is important to design the diameter of the feed pipe so as to be of a sufficiently large size to facilitate the flow of the backwashing wastewater. However, this in itself is an impediment to membrane filtration equipment compact and also making the detracts from the degree of flexibility that is desirable in the arrangement of the equipment. In actual equipment in which a plurality of elements is used as a single operational unit, as is shown in FIG. 5 for example, the feed pipe for enhancing the backwashing waste water to flow is used as a lower header pipe 15, and is directly connected to the lower portion of the casing accommodating the membrane element 1. In this structure, in order to enable the backwashing wastewater flowing from the plurality of elements to move quickly, both the lower header pipe 15 and the backwashing waste water valve 16 have been required to have large diameters.
Patent Exhibit 1: Japanese Laid-Open Patent Publication No. 2002-126468: Claim 1, Paragraph (0010) Patent Exhibit 2: Japanese Laid-Open Patent Publication No. 2002-35748: Claim 2, Paragraph (0007)